PROJECTSUMMARY/ABSTRACT Immune checkpoint blockade (ICB) using antibodies that disrupt PD1 or CTLA4 signaling can lead to durable responsesinawidevarietyofhumancancers.Unfortunately,onlyaminorityofpatientsobtainclinicalbenefit from ICB. A high non-synonymous nuclear mutation burden has been correlated with increased likelihood of response to ICB, suggesting that mutation induced neoantigens may underlie the tumor associated activation ofimmunecheckpoints.Althoughthishypothesislinkingmutationburdentopeptideneoantigensiscompelling, ithasnotbeencompletelyvalidated.Moreover,elevatedmutationburdendoesnotalwaysdictateresponseto ICB,withsomelowmutationtumorssuchasthyroidcancersshowingresponses.Thus,therearehugegapsin our knowledge of the underlying mechanisms dictating response to ICB. We hypothesize that a high background somatic mutation burden, through introduction of mutations in DNA polymerases, will alter the immune landscape of cancers through the presence of truncal, tumor specific ?neoantigens? that confer responsetoICB.Wewilltestthishypothesisanddecipherunderlyingmechanismsusingnovelmousemodels, This will be approached through the following specific aims: Aim 1: Determine the nature of the immune response to cancers with endogenous high nuclear mutation burden arising spontaneously in mice harboring germline proofreading mutations in Pold1 and Pole and patient tumors harboring POLE and POLD1 mutations.;? Aim 2: Determine how a high nuclear mutation burden affects the growth of oncogene-driven autochthonouscancermodelsandsensitivitytoimmunecheckpointtherapy;?Aim3:Determineifproofreading mutationsinPolgandelevationofmitochondrialgenomemutationburdencontributestoananti-tumorimmune response.. Aim 4: Determine how combining targeted therapy and immune checkpoint therapy are modulated by presence of high background mutation burden. To address these aims, we will utilize mouse models in whichahighbackgroundmutationrateisinducedthroughgermlineproofreadingmutationsinPoleandPold1 (for nuclear mutations) and Polg (for mitochondrial mutations. We will compare how specific oncogenes (Braf inskinandlung,andKrasinlung)inducetumorsineitherwildtypemiceorinmicewithgermlineproofreading polymerasemutationstodeterminehowhighbackgroundmutationratealterstumorgrowth,localandsystemic immune response, and response to targeted therapy and ICB. Our overall goal is to develop a better understanding of how mutation burden affects the immune response to solid tumors. Robust models of high mutation burden cancers can be used to guide rational development of combination treatment strategies that canbetranslatedintotheclinic.